The MDC is a simple thread-specific hash table,
in which the application can stuff values under certain keys and retrieve them later via the "%X{key}" placeholder in Log::Log4perl::Layout::PatternLayouts.

Please note that all of the methods above are class methods,
there's no instances of this class.
Since the thread model in perl 5.8.0 is "no shared data unless explicetly requested" the data structures used are just global (and therefore thread-specific).